An attention is paid to the carbon fiber in a field of batteries (for example, a lithium-ion battery and an electric double-layer capacitor) and fuel cells. In particular, an attention is paid to the carbon fiber nonwoven fabric as an electrode material of the aforementioned batteries. The aforementioned nonwoven fabric is configured of the carbon fiber of which a fiber diameter is 10 μm or so.
Recently, the nonwoven fabric configured of the carbon fiber of which the fiber diameter is 10 μm or less (for example, 1 μm 0 or so) has been required from a viewpoint of an augment in a surface area.
The carbon nanotube produced with a vapor growth method or an arc electric discharge method is known as the carbon fiber having a fine fiber diameter. A fiber length of the carbon nanotube, however, is short. For example, it is 10 # m or less. In addition, the carbon nanotube is expensive. Thus, an application of the carbon nanotube to the electrode material causes a problem.
From such a background, the carbon fiber produced with a melt blow method or an electrospinning method has been proposed.
For example, the method of spinning thermoplastic containing a carbon source (for example, pitch etc.) with the melt blow method, and thereafter, thermally decomposing, carbonizing and graphitizing the aforementioned thermoplastic has been proposed (Patent literature 1 and Non-patent literature 1). In accordance with this method, the carbon fiber having a fine fiber diameter is obtained. However, it is difficult to control the fiber diameter with the melt blow method. The carbon fiber obtained with the melt blow method is large in deviation of the fiber diameters
The method (the electrospinning method) of electrospinning a solution having the carbon source (for example, a polymer such as polyacrylonitrile) dissolved therein, and thereafter, carbonizing and graphitizing it has been proposed (Patent literatures 2 to 5 and Non-patent literature 2). The carbon fiber obtained with this method is small in deviation of the fiber diameters. However, in the method described in the above-mentioned Patent Literatures 2 to 5, the carbon source has to be dissolved in a solvent. By the way, hard pitch and mesophase pitch are high in a graphitization degree. Thus, the hard pitch and the mesophase pitch are preferably employed as the carbon source. However, the hard pitch and the mesophase pitch are not dissolved in the solvent. Thus, the hard pitch and the mesophase pitch are not employed as the carbon source in the above-mentioned Patent Literatures. In the Patent literature 5, carbonization and the graphitization are performed with microwave heating after the electro spinning. Herein, carbon black is essential. The carbon black can be employed as the carbon source. However, the carbon black, similarly to polyacrylonitrile, is low in the graphitization degree. For this reason, only the carbon fiber of which the graphitization degree is low can be obtained.
The technology of performing the electrospinning with the pitch kept in a molten state and thereafter, carbonizing and graphitizing it has been proposed (Patent literature 6).
The carbon fiber obtained with this method is small in deviation of the fiber diameters. And yet, the graphitization degree is high. However, only the carbon source of which the graphitization degree is high is employed in this technology, differently from the above-mentioned technologies. For this reason, shrinkage is small at the time of the carbonization and the graphitization. Thus, it is difficult to obtain the carbon fiber of which the fiber diameter is 1 μm or less. In addition, only soft pitch of which a melting point is 300° C. or lower is employed in the technology of the Patent literature 6. That is, the high pitch and the mesophase pitch of which the melting point is 300° C. or higher cannot be used. In principle, only the carbon fiber of which the surface is flat can be obtained in this method. That is, the carbon fiber having the characteristics of the present invention cannot be obtained.